In a cable system, a network of interconnected electrical cables, referred to as a cable plant, is commonly used to deliver information to subscribers. A cable plant enables a broadband transmission of signals, such as television signals, from a head end facility to a multitude of home receivers. A broadband coaxial cable is advantageously used in this application because it supports a wide range of frequencies and provides signal shielding at a moderate cost in comparison to other media. The wide frequency bandwidth permits definition of a substantial number of information channels on the coaxial cable, thus allowing simultaneous broadcasting of many channels.
Cable systems have, in recent years, moved beyond merely broadcasting television signals over the cable to subscribers in their homes. Subscribers of a cable network nowadays have a transceiver, or a modem, which allows the transmission of digital signals upstream toward the head end of the network. Among many services afforded by cable modems are: an Internet service, a home shopping service using a television catalogue, and a voice-over-IP phone service.
In bidirectional cable networks, the upstream and the downstream signals occupy separate frequency bands called upstream and downstream spectral bands. In the United States, the upstream spectral band typically spans from 5 MHz to 42 MHz, while the downstream spectral band typically spans from 50 MHz to 860 MHz. Downstream information channel signals co-propagate in the downstream spectral band, and upstream signals co-propagate in the upstream spectral band. The frequency separation of the upstream and the downstream signals allows bidirectional amplification of these signals propagating in a common cable in opposite directions.
To provide upstream communication capability to a multitude of subscribers, the upstream frequency channels are used in a so-called time-division multiplexing (TDM) mode. Each cable modem is assigned a time slot, within which it is allowed to transmit information. The lime slots are assigned dynamically by a cable modem termination system (CMTS) disposed at the head end. The time slot information is communicated to individual cable moderns via an allocated downstream channel. Subscribers access available network resources by using a data communication bridge established between CMTS and individual cable modems. Subscribers send data from their digital devices (such as personal computers, televisions, voice-over-IP telephones) into cable modems, which then relay the data to the CMTS. The CMTS, in turn, relays the information to the appropriate network elements. Information destined to the subscriber digital device is provided from the network elements to the CMTS, which in turn relays the information to individual cable modems. The cable modems then relay the information to the digital devices used by the subscribers.
One popular communication standard for bidirectional data transport over a cable network is the Data Over Cable Service Interface Specification (DOCSIS). DOCSIS establishes rules of communication between CMTS and cable modems in a cable network. Three revisions currently exist for a North American DOCSIS standard, DOCSIS 1.x, 2.0, and 3.0. In addition to the 6-MHz wide North American based DOCSIS standard, there exists a European (Euro-DOCSIS) standard formatted for 8-MHz wide bandwidth channels.
As cable communication systems grow and become more complex, the task of proper system maintenance and troubleshooting becomes more and more difficult. The difficulty results from a random nature of signal bursts from individual cable modems. Although the cable moderns are allocated time slots in which they are allowed to transmit, the actual transmission depends on network activity of individual subscribers. Furthermore, the upstream signal bursts from cable moderns have a very short duration and arrive intermittently from a multitude of locations in the cable network. Consequently, an upstream signal from a faulty location is interspersed with upstream signals from other locations. From the troubleshooting standpoint, it is important to identify faulty upstream bursts and the particular anomalous or faulty network location the faulty bursts came from. Therefore, the upstream burst troubleshooting equipment must possess a capability to determine geographical location of a cable modem or modems generating faulty upstream bursts.
Various systems have been devised to maintain and troubleshoot upstream communications in a cable network. Volpe et al. in US Patent application publication 2005/0047442, incorporated herein by reference, disclose a method and an apparatus for quantifying upstream communication signals transmitted by a remotely deployed cable modem. Referring to FIG. 1, a signal integrity analyzer 100 of Volpe et al. includes upstream/downstream diplex filter 28, signal couplers 30, a downstream tuner and demodulator 32, an upstream tuner and demodulator 34, a DOCSIS processor 36, and a portable computer 40 having a storage medium 44. The signal integrity analyzer demodulates a downstream signal carrying information about upstream signal time slots, which is then processed in the DOCSIS processor 36 to extract time slot information for various cable modems, not shown. The portable computer 40 is used to filter the cable modem information and program the US tuner and demodulator 34 to capture upstream bursts arriving in time slots corresponding to the cable modems of interest. The captured upstream bursts are then analyzed for signal distortions, decoding errors, and other faults.
Danzig et al. in U.S. Pat. No. 7,372,872, incorporated herein by reference, disclose a field-programmable gate array (FPGA) implemented network monitor for monitoring downstream and upstream traffic in a cable network. The network monitor of Danzig et al. has a functionality to fully analyze downstream signal information, obtain upstream signal time slots, and capture upstream bursts within the time slots corresponding to cable modems of interest.
Azenko et al. in US Patent Application Publication 2008/0089399, incorporated herein by reference, disclose a “sniffer” device having two cable modems, one to capture downstream data bursts and the other to capture downstream messages and to recover the downstream symbol clock and generate an upstream reference clock which is phase coherent with the recovered downstream symbol clock. The reference clock is used by a cable modern termination system to capture upstream bursts.
The prior-art approaches described above require complex and expensive equipment for receiving, demodulating, decoding, and analyzing both downstream and upstream signals, as well as complex processing circuitry for selecting and processing upstream bursts of interest based on the analyzed downstream time slot information. Accordingly, it is a goal of the present invention to provide a simple and inexpensive method and apparatus for identifying and processing upstream signal bursts in a DOCSIS cable network, which do not require an a priori knowledge of upstream signals timing for proper operation in the upstream domain.